RESEARCH NOTE pubs.acs.org/IECR
Potential of Silicon Carbide-Derived Carbon for Carbon Capture S. K. Bhatia* and T. X. Nguyen School of Chemical Engineering, The University of Queensland, Brisbane QLD 4072, Australia ABSTRACT: Experiments of H2O and N2 adsorption in silicon carbide-derived carbon (SiC-CDC) are reported, showing only weak adsorption of N2, with H2O adsorption being highly kinetically restricted due to the strong hydrophobicity of this material. Such results suggest that SiC-CDC is an attractive option for adsorptive CO2, as our prior experiments have shown much more significant adsorption of this gas. This is confirmed by grand canonical Monte Carlo simulations, which show that in a 2-stage adsorption system the CO2 mole fraction can be increased from 15% (typical of flue gas) to 80 99.6% for the pore sizes that comprise the SiC-CDC.
’ INTRODUCTION The capture of CO2 from flue gas for sequestration or other use is currently the subject of much worldwide attention and has been recognized as a viable near-term option for greenhouse gas mitigation. While chemical absorption1 in amines is the most mature technology, the toxicity of such solvents is an issue that makes this an environmentally unattractive option. In contrast, the selective adsorption of CO2 on a suitable adsorbent, if achievable, offers a cleaner, more environmentally friendly, and perhaps more economical alternative. However, the effect of the coadsorption of H2O, which saturates flue gas, is critical, since adsorbed H2O can significantly degrade the performance of any process for CO2 capture.2 4 While CO2 is strongly adsorbed on most adsorbents such as zeolites,3,5,6 metal organic frameworks (MOFs),7 and carbons,8 pure component H2O isotherms show gravimetric adsorbed amounts that are comparable to or even higher than those of CO2.3,6,7 In the case of zeolites and MOFs the surfaces are intrinsically polar, due to the framework constitution, and adsorb H2O, but in the case of carbons, which are inherently hydrophobic, H2O adsorption is believed to be due to the presence of numerous polar carboxyl, OH, or epoxide-like group-terminated surface-sites4,9 The polar surface sites have high affinity for H2O, and the initial H2O molecules adsorbed on such sites serve as nuclei for the growth of H2O clusters due to strong intramolecular interactions, particularly electrostatically mediated H-bonding.9 In contrast to conventional carbons made from natural precursors, carbide derived carbons (CDCs),8,10 13 being synthesized from an inorganic source, have no polar functional groups and are composed of purely covalently bonded carbon. Depending on their structure, such carbons may retain the intrinsic hydrophobic nature of carbon, and therefore may not be adsorbers of water. If this is true, then such CDCs may be a good alternative for CO2 capture, as the main constituent of flue gas, nitrogen, is only weakly adsorbing, while H2O has an unfavorable interaction with CO2.14 Thus, the adsorption of CO2 will not induce cooperative effects through electrostatic interactions with H2O. In particular, SiC derived carbon has significant sp3 bonding, inherited from the diamond-like structure of the precursor (SiC),8,12 and is an attractive prospect in view of the high hydrophobicity associated with such bonding.15 r 2011 American Chemical Society
Figure 1. Pore-size distribution of SiC-CDC synthesized at 600 °C.
Our recent work8,12 has shown strong adsorption of CO2 on SiC-based CDC. Here we experimentally demonstrate that both H2O and N2 are very weakly adsorbed on carbon derived from SiC. Simulations are also conducted for the adsorption of a CO2/ N2/O2 mixture in carbon slit pores, showing the feasibility of CO2 capture in hydrophobic carbon.
’ EXPERIMENTAL SECTION The adsorption of H2O and of N2 at 298 K on SiC-based CDC was investigated using samples prepared by chlorination of SiC in this laboratory.8,12 In our earlier characterization studies, carbon samples prepared at various temperatures in the range of 600 1000 °C had been investigated and showed increasing degree of short-range ordering with increase in temperature. For the present pilot study, samples synthesized at 600 °C were chosen, as it was expected that the lower degree of graphitization would lead to a larger fraction of sp3 bonding and increased hydrophobicity. Figure 1 depicts the pore size distribution of the 600 °C SiC-CDC, obtained12 by our finite wall thickness Received: May 22, 2011 Accepted: August 8, 2011 Revised: July 14, 2011 Published: August 08, 2011 10380
dx.doi.org/10.1021/ie201094d | Ind. Eng. Chem. Res. 2011, 50, 10380–10383
Industrial & Engineering Chemistry Research
Figure 2. Experimental and simulated nitrogen isotherms on SiCderived carbon prepared at a chlorination temperature of 600 K.
nonlocal density functional theory interpretation of 87 K Ar adsorption data, showing a strong peak at about 0.46 nm width (defined as Hin = Hcc 0.34 nm, where Hcc is the center center spacing of the pore walls and 0.34 nm is the approximate diameter of a carbon atom), around which the bulk of porosity lies, and smaller peaks at about 0.74 and 1.1 nm. There is very little porosity associated with pores larger than 1.5 nm in width. The H2O and N2 adsorption measurements on the sample were conducted using a Micromeritics ASAP 2020 volumetric adsorption analyzer equipped with a water vapor sorption accessory. Simulation. The pure component nitrogen isotherm was determined in the grand canonical Monte Carlo (GCMC) simulation at a representative pore size of Hcc = 0.9 nm (i.e., Hin = 0.56 nm), to compare with the experimental result. Mixture adsorption simulations for CO2/N2/O2 at 300 K, in carbon slit pores having sizes in the range of 0.75 1.5 nm pore width (carbon center center distance between opposing surfaces, Hcc) were also conducted in the grand canonical ensemble. These simulations mimicking the μ, V, T ensemble used the established Metropolis sampling scheme16 for moving (including rotation), creating, or deleting molecules. A bulk gas pressure of 1 bar was chosen for all simulations, and at this pressure and the temperature of 300 K the gas was considered ideal for the purpose of the present calculations. To mimic the composition of flue gas we used partial pressures of 0.15 bar for CO2, 0.8 bar for N2, and 0.05 bar for O2. Starting from an empty simulation box, each simulation run sampled 5 106 configurations, of which the first 0.5 106 configurations were used to achieve equilibration and were rejected. A 3-center potential model was used for CO2. The potential parameters used were those established by Nguyen et al.,17 with the values σoo = 0.3026 nm, σcc = 0.2824 nm, εoo/k = 82 K, εcc/k = 28.68 K, and point charges on the atom centers of qo/e = 0.332 and qc/e = 0.664. The O O distance, doo is 0.2324 nm and the C O distance is 0.1162 nm. For N2 we used the multisite model of Schneemilch et al.,18 comprising two Lennard-Jones (LJ) sites and four Coulomb sites. The LJ sites are located at (0.0547 nm, with partial charges of 0.373e at (0.0847 nm, and 0.373e at (0.1044 nm. The LJ sites had the potential parameters εNN/k = 36 K, σNN = 0.33 nm. For O2, which has a weak quadrupole, following Sokolowski,19 we used a two-site LJ model with εοο/k = 54.3 K, σοο = 0.305 nm, and an O O distance of 0.12078 nm. The Lorentz Berthelot rules were used to estimate parameters for the atom atom cross interactions. The interaction potential between
RESEARCH NOTE
Figure 3. H2O adsorption isotherm at 298 K, in SiC-CDC synthesized at 600 K. Inset depicts the H2O isotherm23 on activated carbon fiber, ACF-15, showing much larger adsorption.
an adsorbate LJ center and each wall in the graphitic slit-pore was represented by the Steele20 10 4 3 potential, with εcc/k = 28 K, σcc = 0.34 nm, and a carbon atom density of 114 nm 3. A cutoff distance of 1.5 nm was used, and the simulation box size varied between 5 and 25 nm, usually containing about 250 350 particles. Periodic boundary conditions were used.
’ RESULTS AND DISCUSSION Figure 2 depicts the 298 K nitrogen isotherm on the SiC-CDC sample, obtained using the ASAP 2020 analyzer, as well as that obtained by GCMC simulation using a pore width of 0.9 nm (Hcc). The micropore volume of the SiC-CDC is taken as 0.61 cm3/g and is estimated based on the densities of SiC and graphite. Both simulation and experiment agree remarkably well, despite the use of single representative pore width, illustrating the ability of the simulation to predict adsorption in the SiCCDC, despite the presence of significant sp3 bonding and deviation from graphitic structure. Molecular models based on density functional theory have already been shown to adequately predict adsorption behavior of CO2 and CH4 in SiC-CDC and Ti3SiC2,8,12,13 hence this agreement is not unexpected. In Figure 1, very weak adsorption of N2 is seen, with an adsorbed amount of 0.42 mmol/g at 1 bar pressure. In contrast, CO2 adsorbs much more strongly, with an adsorbed amount of 2.5 3 mmol/g, depending on synthesis conditions.12 Similar results for CO2 adsorption have been found earlier for Ti3SiC2-derived carbon,13 with small variations in the amounts adsorbed between different synthesis temperatures and subsequent heat treatment times. Perhaps more remarkable is the H2O isotherm at 298 K, which shows a very small amount of adsorption that appears to depend on equilibration time. Figure 3 depicts this isotherm, determined for a very long equilibration time (8 h per point, on an average) and a shorter equilibration time (1 h per point, on an average). A clear time dependency is observed, with the longer equilibration point showing higher amounts adsorbed, reaching up to 1.5 mmol/g after about 10 days of adsorption (over the whole run). On the other hand, at the shorter equilibration time averaging about 1 h per point only an adsorbed amount of 0.8 mmol/g is reached. These results clearly indicate kinetic restrictions to water entry at pore mouths and that the isotherms are not representative of true equilibrium. Further evidence of such restrictions is offered by the desorption data in Figure 3, which shows strong hysteresis with only a small amount of 10381
dx.doi.org/10.1021/ie201094d |Ind. Eng. Chem. Res. 2011, 50, 10380–10383
Industrial & Engineering Chemistry Research
Figure 4. Variation of (a) composition of the adsorbed phase with pore width in carbon slit pores, in the second stage of a 2-stage adsorption system at 300 K, and (b) adsorbed densities with pore size, in each stage. Open symbols represent the first stage, and closed symbols represent the second stage.
desorption until a relative pressure of 0.3 is attained. On the other hand the adsorption data shows a nearly linear increase in amount adsorbed with pressure. This strong kinetic restriction to water entry is somewhat peculiar to the SiC-CDC, as much larger water adsorption has been noted for other hydrophobic carbons having little or no polar surface groups,21 23 with amounts adsorbed reaching as much as 25 50 mmol/g in practical time-scales. The inset in Figure 3 depicts the data of Sullivan et al.23 for water adsorption in an activated carbon fiber, ACF-15, at 298 K, showing adsorption exceeding 25 mmol/g. This carbon fiber has a surface O/C atomic ratio of only 0.028, and is essentially hydrophobic, given the report that varying the O/C atomic ratio in the range of 0.018 0.074 has little or no effect on water adsorption.24 The noticeable higher adsorption of H2O in ACF-15 is remarkable, given that its pore volume of 0.54 cm3/g is slightly smaller than that of SiC-CDC of 0.61 cm3/g. Some hysteresis has been reported for an activated carbon fiber that had been stripped of most of its polar surface groups;22 however, complete pore-filling was observed on adsorption, suggesting mild kinetic restrictions on desorption but not on adsorption. Such differences in kinetic barriers between adsorption and desorption have been predicted by us for other gases such as CH4 using transition state theory25 based on atomistic models of carbons, and are due to the differences in free energy of the adsorbed molecules at the binding sites in the pores or cages on either side of the window connecting them. Indeed, we have reported mild inaccessibility for CH4, and to a lesser extent for CO2, in SiC-CDC as well as Ti3SiC2 DC at near ambient temperatures.22,23 This inaccessibility depends on synthesis temperature and is reduced on postsynthesis heat treatment, indicating improved short-range structural ordering at
RESEARCH NOTE
constrictions near pore entrances. Clearly, further studies of water adsorption in Si-CDC, involving effects of synthesis conditions and heat treatment time, are required to identify synthesis conditions that most strongly inhibit water adsorption. The above observations of dramatically slow water adsorption suggest that SiC-CDC may offer an attractive alternative for CO2 capture. In particular small amounts of preadsorbed H2O of 2 6 wt % have been shown not to affect CO2 isotherms in MOFs,7 which may be attributed to their unfavorable interaction.14 Indeed, in our prior experiments8,12 the CO2 uptake in SiCCDC is significantly more rapid, and a near equilibrium separation, involving adsorption times much smaller than those required for significant water adsorption, may be expected to be feasible. To investigate this possibility, as a representative example of flue gas adsorption, we conducted GCMC simulations of the adsorption of a 15% CO2, 80% N2, 5% O2 mixture at 1 bar total pressure, and 300 K in carbon slit pores of various widths. It was found that in a pore of width 0.9 nm (Hcc), typical of an activated carbon, the CO2 content in the adsorbed phase increased to about 67%. If the adsorbed gas is completely desorbed and subsequently subjected to a second adsorption step in a 0.9 nm pore at 1 bar, the CO2 content increased to 97%. Figure 4a depicts the variation of the mole fraction CO2 with pore width, in the adsorbed phase in the second stage. It is seen that this CO2 content varies between about 80% and 99.6%, over the pore width range of 0.41 1.16 nm, which covers the range of pore sizes pertinent to the SiC-CDC, depicted in Figure 1. At the same time the N2 content varies between 0.2 and 15%, while the O2 content varies between 0.2 and 4.7%. Figure 4b depicts the variation of adsorbed densities of the different species with pore width, showing considerable adsorption of CO2 even in the first stage. It is evident that small pore sizes of about 0.4 0.5 nm are preferable both from CO2 amount adsorbed as well as CO2 selectivity point of view. This is precisely the range predominant in the SiC-CDC, as seen in Figure 1. These results are based on the premise that the water that saturates the flue gas will adsorb only to an insignificant extent in the hydrophobic carbon during the time scale of the adsorption step in the first stage. This would appear to be reasonable, given the observations of very small water adsorption even over several hours of exposure to saturated water vapor. Similar slow adsorption of H2O in SiC-CDC has been reported by Gupta,26 with about 30 h needed for equilibration of water vapor at 100% relative humidity. Further, while we have illustrated the concept through simulations at 1 bar total pressure, our simulations show that higher pressures of as much as 10 bar may also be used to increase the adsorbed phase density and reduce the amount of adsorbent without affecting the separation efficiency. It is to be noted that in practice postcombustion flue gas will be usually available at 40 60 °C after desulfurization, and will be saturated with about 10% moisture. However, we do not expect this to significantly affect moisture adsorption, given the extremely slow kinetics of its adsorption. In recent work27 we have shown that water adsorption in disordered carbons is governed by the formation of sufficiently large hydrogen-bonded clusters at interconnections between neighboring pores. The activation energy for the breakage of hydrogen bonds in bulk water is about 40 kJ/mol,28 which suggests an increase in water adsorption rate by a factor of only about 2 on increasing temperature to 313 K. Thus, we anticipate that even at temperatures of about 40 °C capture of SiC-CDC should be feasible, but this needs to be confirmed by experiments at higher temperatures. At the same time, we anticipate only minor reduction in CO2 adsorption at the 10382
dx.doi.org/10.1021/ie201094d |Ind. Eng. Chem. Res. 2011, 50, 10380–10383
Industrial & Engineering Chemistry Research higher temperature, based on our prior experimental studies.8,12 Higher temperature studies along these lines are underway in our laboratory and will be reported in a more detailed article in due course. Even otherwise, we expect there are several niche or small scale applications where cooling of the desulfurized gas to 300 K can be accomplished without significant energy penalty, if chilling facilities are already available. While water adsorption in a single cycle will be very small when using SiC-CDC, it may be expected that there will be some water accumulation in the adsorbent, and in time the capture efficiency will reduce and the adsorbent will need to undergo regeneration through thermal water desorption. The number of operational cycles between regenerations is still unknown, and needs more detailed studies. Finally, we note that while we have targeted the SiC-CDC in view of its structural uniqueness, and presence of significant sp3 bonding which imparts it strong hydrophobic character, other CDCs, such as TiC-CDC, may also offer scope for CO2 capture due to their favorable properties for CO2 adsorption.29 The key to their success is likely to be the kinetics of water adsorption in their structures, and whether it is extremely slow as in SiC-CDC.
’ CONCLUSIONS On the basis of the above simulations it would appear that SiCCDC is a potential candidate as an adsorbent for CO2 capture, with its hydrophobicity deterring water adsorption. While experimental confirmation is awaited, the above success of the GCMC simulation of nitrogen adsorption (albeit using a representative pore width), and of our prior density functional theorybased predictions for CO2 and CH4 adsorption,8,12,13 suggests reliability of the simulation-based results. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT The authors thank an anonymous reviewer for bringing to their attention ref 29 which appeared while this article was under review. ’ REFERENCES (1) Puxty, G.; Rowland, R. Modeling CO2 Mass Transfer in Amine Mixtures: PZ-AMP and PZ-MDEA. Environ. Sci. Technol. 2011, 45, 2398. (2) Li., G.; Xaio, P.; Webley, P. Binary Adsorption Equilibrium of Carbon Dioxide and Water Vapour on Activated Alumina. Langmuir 2009, 25, 10666. (3) Li., G.; Xaio, P.; Webley, P. Competition of CO2/H2O in Adsorption Based CO2 Capture. Energy Procedia 2009, 1, 1123. (4) Rutherford, S. Probing the Mechanism of Water Adsorption in Carbon Micropores with Multitemperature Isotherms and Water Preadsorption Experiments. Langmuir 2006, 22, 9967. (5) Rege, S.; Yang, R.T. A Novel FTIR Method for Studying Mixed Gas Adsorption at Low Concentrations: H2O and CO2 on Nax and -Alumina. Chem. Eng. Sci. 2001, 56, 3781. (6) Wang, Y.; LeVan, M. D. Adsorption Equilibrium of Carbon Dioxide and Water Vapour on Zeolites 5A and 13X and Silica Gel: Pure Components. J. Chem. Eng. Data 2009, 54, 2839. (7) Liu, J.; Wang, Y.; Benin, A. I.; Jakubczak, P.; Willis, R. R.; LeVan, M. D. CO2/H2O Adsorption Equilibrium and Rates on Metal Organic Frameworks: HKUST-1 and Ni/DOBDC. Langmuir 2010, 26, 14301.
RESEARCH NOTE
(8) Bonilla, M. R.; Nguyen, T. X.; Bae, J.-S.; Bhatia, S. K. Heat TreatmentInduced Structural Changes in SiC-Derived Carbons and Their Impact on Gas Storage Potential. J. Phys. Chem. C 2010, 114, 16562. (9) Hantal, G.; Picaud, S.; Hoang, P. N. M.; Voloshin, V. P.; Medvedev, N. N.; Jedlovsky, P. Water Adsorption Isotherms on Porous Onion-like Carbonaceous Particles. Simulations with the Grand Canonical Monte Carlo Method. J. Chem. Phys. 2010, 133, 144702. (10) Gogotsi, Y.; Nikitin, A.; Ye, H.; Zhou, W.; Fischer, J. E.; Yi, B.; Foley, H. C.; Barsoum, M. W. Nanoporous Carbide-Derived Carbon with Tunable Pore Size. Nat. Mater. 2003, 2, 591. (11) Yushin, G.; Dash, R.; Jagiello, J.; Fischer, J. E.; Gogotsi, Y. Carbide-Derived Carbons: Effect of Pore Size on Hydrogen Uptake and Heat of Adsorption. Adv. Funct. Mater. 2006, 16, 2288. (12) Nguyen, T. X.; Bae, J.-S.; Bhatia, S. K. Characterization and Adsorption Modelling of Silicon Carbide-Derived Carbons. Langmuir 2009, 25, 2121. (13) Bae, J.-S.; Nguyen, T. X.; Bhatia, S. K. Influence of Synthesis Conditions and Heat Treatment on the Structure of Ti3SiC2-Derived Carbons. J. Phys. Chem. C 2010, 114, 1046. (14) Billemont, P.; Coasne, B.; De Weireld, G. An Experimental and Molecular Simulation Study of the Adsorption of Carbon Dioxide and Methane in Nanoporous Carbons in the Presence of Water. Langmuir 2011, 27, 1015. (15) Paul, R.; Das, S. N.; Dalui, S.; Gayen, R. N.; Roy, R. K.; Bhar, R.; Pal, A. K. Synthesis of DLC Films with Different Sp2/Sp3 Ratios and their Hydrophobic Behaviour. J. Phys. D.: Appl. Phys. 2008, 41, 055309. (16) Allen, M. P.; Tildesley, D. J. Computer Simulation of Liquids; Clarendon: Oxford, 1987. (17) Nguyen, T. X.; Bhatia, S. K.; Nicholson, D. Prediction of High Pressure Adsorption Equilibrium of Supercritical Gases Using Density Functional Theory. Langmuir 2005, 21, 3187. (18) Schneemilch, M.; Bellarosa, L.; Nakai, K.; Quirke, N. Comparison of Nitrogen and Carbon Dioxide as Molecular Probes of Low Surface Area Carbonaceous Materials. Surf. Sci. Catal. 2007, 160, 365. (19) Sokolowski, S. Adsorption of Oxygen in Slit-like Pores—Grand Canonical Ensemble Monte-Carlo Studies. Mol. Phys. 1992, 75, 999. (20) Steele, W. A. Physical Interaction of Gases with Crystalline Solids. 1. Gas-Solid Energies and Properties of Isolated Adsorbed Atoms. Surf. Sci. 1973, 36, 317. (21) Ohba, T.; Kaneko, K. Initial Filling Mechanism of Predominant on Water Adsorption of Hydrophobic Slit-Shaped Carbon Nanopores. J. Phys. Conf. Ser. 2009, 177, 012001. (22) Kimura, T.; Kanoh, H.; Kanda, T.; Ohkubo, T.; Hattori, Y.; Higaonna, Y.; Denoyel, R.; Kaneko, K. Cluster-Associated Filling of Water in Hydrophobic Carbon Micropores. J. Phys. Chem. B 2004, 108, 14043. (23) Sullivan, P. D.; Stone, B. R; Hashisho, Z.; Rood, M. J. Water Adsorption with Hysteresis Effect onto Microporous Activated Carbon Fabrics. Adsorption 2007, 13, 173. (24) Ohba, T.; Kanoh, H.; Kaneko, K. Cluster-Growth-Induced Water Adsorption in Hydrophobic Carbon Nanopores. J. Phys. Chem. B 2004, 108, 14964. (25) Nguyen, T. X.; Bhatia, S. K. Kinetic Restriction of Simple Gases in Porous Carbons: Transition State Theory Study. Langmuir 2008, 24, 146. (26) Gupta, P. Water Vapor Adsorption onto Nanostructured Carbide Derived Carbon (CDC). Ph.D. Thesis, University of Illinois, Chicago, 2008. (27) Nguyen, T. X.; Bhatia, S. K. How Water Adsorbs in Hydrophobic Nanospaces. J. Phys. Chem. C, published online July 19, 2011, DOI: 10.1021/jp2053162. (28) Starr, F. W.; Nielsen, J. K.; Stanley, H. E. Fast and Slow Dynamics of Hydrogen Bonds in Liquid Water. Phys. Rev. Lett. 1999, 2294–2297. (29) Presser, V.; McDonough, J.; Yeon, S.-H.; Gogotsi, Y. Effect of Pore Size on Carbon Dioxide Sorption by Carbide Derived Carbon. Energy Env. Sci. 2011, 4, 3059–3066. 10383
dx.doi.org/10.1021/ie201094d |Ind. Eng. Chem. Res. 2011, 50, 10380–10383
